🚀 go-pugleaf

>Date: Thu, 20 Jun 1991 15:45:02 -0700 (PDT)
>From: Steve Hubert <hubert@cac.washington.edu>
>Sender: Steve Hubert <hubert@kamba.cac.washington.edu>
>Subject: backplane backbone, routing or bridging?
>To: cisco@spot.colorado.edu
>
>We are about to connect all N (N ~ 20) of our routers to a single AGS+ in
>order to use the AGS+ backplane as our "backbone subnet".  There will be point
>to point ethernet between each router and the backbone router.  We carry only
>IP traffic over the backbone.  It occurs to us that, instead of assigning a
>subnet number to each of these point to point links, we could use the AGS+ as
>a bridge and use only one subnet number instead of N.  I don't think this has
>anything to do with the normal bridge-vs-router debate.  This would be an N-
>port bridge with a single router attached to each port.  Does anyone have any
>opinions on advantages/disadvantages of the two approaches?  For example, is
>one way faster than the other?  Thanks.
>
>Steve Hubert
>Networks and Distributed Computing, Univ. of Wash., Seattle
>hubert@cac.washington.edu
>
>

You may not be able to do this.  You cannot both route and bridge the
same protocol in a cisco chassis, even if you only route on some interfaces
and only bridge on others.  In other words, if you want to route IP in
either the AGS+ chassis, you cannot bridge IP in it.  I wouldn't do it
anyway, as you lose the chance of gathering all that per-protocol
information in the AGS+ (such as IP accounting).  If you want to bridge,
look into getting a genuine multiport bridge (A.K.A. fast packet switch).
This would undoubtedly save you money over the AGS+ if that's all you
plan to do with it.

However you do not have to assign a whole subnet to each point-to-point
ethernet.  You can have a single subnet which is shared by all the
point-to-point ethernets on both ends.  Assuming you subnet a class B
on an 8-bit boundary, and the "point-to-point cloud" is subnet 100,
you could define the subnet mask 255.255.255.252 to allow 6 bits
of "sub-subnet" and 2 bits of "host" (0 is reserved, 3 is broadcast,
1 and 2 are the two endpoints.

		AGS+			
	intf	address		Brcast		Router	intf	address
	----	-------		------		------	------	-------
	ether0	XX.YY.100.1 	XX.YY.100.3	AGS 1	ether0	XX.YY.100.2
	ether1	XX.YY.100.5	XX.YY.100.7	AGS 2	ether0	XX.YY.100.6
	ether2	XX.YY.100.9	XX.YY.100.11	AGS 3	ether0	XX.YY.100.10

			...

	ether18	XX.YY.100.77	XX.YY.100.79	AGS 20	ether0	XX.YY.100.78
	ether19	XX.YY.100.81	XX.YY.100.83	AGS 20	ether0	XX.YY.100.83

There you have your 20 ethernet point to point links between the AGS+ and
the 20 other routers, and plenty of address space left in subnet 100 for
more.

	- Alan S. Watt
	  High Speed Networking, Yale University
	  Computing and Information Systems
	  Box 2112 Yale Station
	  New Haven, CT  06520-2112
	  (203) 432-6600 X394
	  Watt-Alan@Yale.Edu


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